Professor Johnsen joined the faculty in 1998, following a distinguished career in Washington, D.C. She served in the U.S. Department of Justice under President Bill Clinton (1993-1998), including as Acting Assistant Attorney General heading the Office of Legal Counsel where she provided constitutional and other legal advice to the Attorney General, the Counsel to the President, and the general counsels of the various executive branch agencies. From 1988-1993, she was the Legal Director of the National Abortion and Reproductive Rights Action League (now NARAL Pro-Choice America). She also worked at the American Civil Liberties Union Reproductive Freedom Project and clerked for Judge Richard D. Cudahy of the U.S. Court of Appeals for the Seventh Circuit.

Johnsen serves on the national board of the American Constitution Society for Law and Policy and as co-chair of the ACS Issue Group on Separation of Powers and Federalism. Her research interests include issues of separation of powers (especially presidential power) and civil liberties (especially reproductive rights). She has testified before Congress and appeared on many national television and radio news programs as an expert on these issues.

Her courses include Constitutional Law, the First Amendment, and Seminars in the Separation of Powers and Sexuality, Reproduction and the Constitution.